You should to add your desired commands to this list. The first element in each pair is the letter you want to press after C-c [ to select the given citation format. The empty square brackets denote optional arguments; RefTeX has variables to control whether it prompts your for those. And %l is where the cite key goes.

(You could also set this variable via M-x customize-variable, but I loathe that particular feature of Emacs.)

I'm not sure what the syntax of \blockcquote is...if it has more non-optional arguments than just a cite key, RefTeX might not be able to fully support it (i.e., it inserts the command but you then have to move backwards to fill out other arguments).

@N.N., your link also says "But it is OK to use eval-after-load in your personal customizations if you don't feel that they must meet the design standards [...] for wider use." Since this code is run at emacs startup, I use eval-after-load to defer loading the reftex-vars library (until the first TeX file is opened), which cuts down on startup time – drastically, if you use eval-after-load consistently. It's a fairly common .emacs idiom AFAIK. It's also possible to use the M-x customize interface for deferred variable setting, but I dislike it (and it can't really be copy/pasted).
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AaronJan 17 '12 at 0:51